util: iptables: use VIR_AUTOFREE instead of VIR_FREE for scalar types
    
    By making use of GNU C's cleanup attribute handled by the
    VIR_AUTOFREE macro for declaring scalar variables, majority
    of the VIR_FREE calls can be dropped, which in turn leads to
    getting rid of most of our cleanup sections.
    
    Signed-off-by: Sukrit Bhatnagar <skrtbhtngr@gmail.com>
    Reviewed-by: Erik Skultety <eskultet@redhat.com>